In a conventional high frequency semiconductor device in which a plurality of field effect transistors (FET) are arranged in parallel, for example, an input dividing/matching circuit is arranged on the input side of the unit FETs, and an output combining/matching circuit is arranged on the output side of the unit FETs. Such a conventional high frequency semiconductor device has gain in a wide bandwidth from a high frequency band to a low frequency band, and oscillators may occur at various frequencies in the device.
Thus, the output end of the input dividing/matching circuit and the input end of the output combining/matching circuit are divided for some unit FETs in the conventional high frequency semiconductor device in order to suppress high frequency oscillations on the GHz order, for example. Resistors (so-called isolation resistors or balance resistors) are provided between the divided output ends and between the divided input ends, respectively.
Further, in the conventional high frequency semiconductor device, low-frequency-oscillation-suppressing-circuits in which an inductor L, a resistor R, and a capacitor C are connected in series are connected to the unit FETs arranged at both ends among the unit FETs arranged in parallel in order to suppress low frequency oscillations on the MHz order occurring in the high frequency semiconductor device, for example.
In the conventional high frequency semiconductor device, however, the divided output ends of the input dividing/matching circuit and the divided input ends of the output combining/matching circuit are connected via the isolation resistors. Thus, there are plurality of isolation resistors between the unit FETs arranged near the center of the unit FETs arranged in parallel, and the low-frequency-oscillation-suppressing-circuits. Thus, a resistor with a high resistance value is formed between the unit FETs arranged near the center and the low-frequency-oscillation-suppressing-circuits, respectively, and the unit FETs arranged near the center are not substantially connected to the low-frequency-oscillation-suppressing-circuits. Therefore, the low-frequency-oscillation-suppressing-circuits work on at least the unit FETs arranged at both ends, but the low-frequency-oscillation-suppressing-circuits do not work enough on the unit FETs arranged at near the center. There is a problem that the low-frequency-oscillation-suppressing-circuits hardly work on the unit FETs arranged near the center and it is difficult to suppress low frequency oscillations.
There is a problem that when the output end of the input dividing/matching circuit and the input end of the output combining/matching circuit are not divided and a plurality of unit FETs are connected to one undivided output end and one undivided input end in order to solve the above problem, high frequency oscillations may occur.
That is, there is a problem that it is difficult to suppress both low frequency oscillations and high frequency oscillations in the conventional high frequency semiconductor device.
If the output ends of the input dividing/matching circuit and the input end of the output combining/matching circuit are divided, respectively, and the low-frequency-oscillation-suppressing-circuits are connected for each unit FETs, both low frequency oscillations and high frequency oscillations can be suppressed. However, an extremely large number of low-frequency-oscillation-suppressing-circuits are required, and thus a size of the high frequency semiconductor device becomes extremely large. Thus, such a solution is not practical.